Arsonium ylides stability

Thus arsonium ylides stabilized by acyl groups alkoxycarbonyl... 

A facile method for the preparation of a variety of stabilized arsonium ylides in good yield has been developed by the action of active methylene compounds with tertiary arsine oxide or tertiary arsine dihalide. Thus triphenyl-arsine dihalides react with a number of active methylene compounds in the presence of a tertiary amine to afford arsonium ylides (6) (40). The reaction of triphenylarsine oxide with active methylene compounds in the presence of either acetic anhydride or triethylamine-phos-phorus pentoxide gave rise to arsonium ylides (6) (32, 36. 65, 67). 

A. Reactions of Stabilized Arsonium Ylides with Carbonyl Compounds... 

Thus the stabilized arsonium ylide carbomethoxymethylene tri-phenylarsorane reacted with ketones forming the thermodynamically more stable olefinic compounds predominantly. This could be explained as follows ... 

Bravo et al. reported that the carbonyl-stabilized arsonium ylides reacted with -hydroxybenzaldehydes to give o-hydroxychalcones (12). 

Nesmeyanov et al. reported that some stabilized and semistabilized arsonium ylides reacted with a few aldehydes, acetone, and cyclohexanone to form corresponding olefins (71). 

The stabilized arsonium ylides reacted either with a,/3-unsaturated diketones or diesters to form corresponding cyclopropanes (72). 

Cavicchio c-Y al. (14) studied the reaction of stabilized ylides and related onium salts with tropylium ion and found that the reaction between a tropylium salt and an arsonium ylide led to tra .v-chalcone (48) along with triphenylarsine. 

Current results indicate that stabilized arsonium ylides such as phenacylide, carbomethoxymethylide, cyanomethylide, fluorenylide, and cyclopentadienylide afford only olefinic products upon reaction with carbonyl compounds. Nonstabilized ylides such as ethylide afford almost exclusively epoxides or rearranged products thereof. However, semi-stabilized arsonium ylides, such as the benzylides, afford approximately equimolar amounts of olefin and epoxide. Obviously, the nature of the carbanion moiety of the arsonium ylide greatly affects the course of the reaction. It is reasonable to suppose that a two-step mechanism is involved in the reaction of heteronium (P, S, and As) ylides with carbonyl compounds (56). 

In comparison to the stabilized phosphonium ylides which fail to react with cv,/3-unsaturated esters or ketones, the corresponding arsonium ylides react smoothly with ,/3-unsaturated esters or ketones (e.g., PhCO—CH=CH—CH3) to form cyclopropanes in fair to good yields. Furthermore, the reaction is stereospecific (45, 45). 

Many arsonium ylides dissolve in acids to form salts, from which they can be re-obtained by treatment with a base, as in the salt method for their preparation. The basicity of the ylides indicates the relative stabilities of the ylides and their salts and in so doing gives some guide to the stability of the ylides. Thus stable ylides are less readily protonated than are reactive ylides and require weaker bases for their generation from salts. 

The general pattern which emerged was that stable arsonium ylides provided alkenes whilst reactive arsonium ylides gave epoxides . This was attributed to stabilization of the transition state leading to alkene formation being provided by those same electron-withdrawing groups which stabilized the ylides ... 

Semi-stabilized arsonium ylides are intermediate in behaviour between stable and reactive ylides, and may provide alkenes and/or epoxides " °. In these cases factors such as the substituent groups on arsenic, and the nature of the solvent and of the base, may become important in determining the nature of the product this will be considered in more detail later (Section V.A.4). Small changes in the structure of the ylidic moiety may also have a marked effect for example, whereas triphenylarsonium jS-napththylmethylide reacts to give epoxide, the presence of a bromine atom at the adjacent a-position of the naphthalene ring results instead in the formation of alkenes . ... 

Semi-stabilized arsonium ylides react with thioketones with cyclic thioketones tri-phenylarsonium benzylides gave only exocyclic alkenes and no thiirans . Benzo-thiopyrones (and, better, benzopyrones) also react to give exocyclic alkenes, in these cases arylidenebenzopy rans ... 

Acylation has frequently been used to convert arsonium ylides into other more stabilized ylides, reagents used being acid chlorides ° acid anhyd-rides " or esters These examples all involve C-acylation but complications can arise, as shown in equation 17 It was suggested that reaction with benzoyl bromide gives a kinetically controlled product which, in the presence of acetate, is converted into the thermodynamically controlled product In this case delocalization of the negative charge leads to acylation at a site other than the ylidic carbon atom. 

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

Arsinimines 35 are the nitrogen analogues of arsonium ylides. They appear to be more resistant to hydrolysis than are ylides, for even the simple non-stabilized example (35,... 

Many arsinimines are stable solids, handleable in air as in the case of arsonium ylides, the presence of electron-withdrawing substituents on the nitrogen atom increases their stability. When heated, N-benzoyltriphenylarsinimine decomposes to give triphenylarsine oxide and benzonitrile . 

Arsonium ylides were discovered near the turn of the century, but their reactions with carbonyl compounds did not become elucidated until the 1960s. In a broad sense, arsonium ylides are midway in chemical behavior between ylides of phosphorus and those of sulfur. Stabilized arsonium ylides react with carbonyl compounds to afford alkenes, whereas the unstabilized analogs give rise to epoxides. More subtly, the nature of the substituents on either the ylide arsenic or carbon atom can alter the course of the reaction the choice of solvent can exert a similar effect. ... 

Semi-stabilized arsonium ylides react with lactones and thiolactones to give products of olefination. For example, the ylide from triphenylbenzylarsonium bromide and sodium hydride reacts with benzo-a-pyrone to give the benzylidene (35) as the exclusive product. Carbonyl-stabilized arsonium ylides fail to condense with benzo-a-pyrone. 

Arenediazonium salts also react with stabilized phosphonium, arsonium, pyridinium, and sulfonium ylides (12.111) in acetonitrile, yielding via the azo-onium salt (12.112) the azo-onium ylide (12.113, yellow to red), and in some cases the for-mazane (12.114) (Froyen and Juvvik, 1992). 

Two different synthetic strategies were proposed to obtain a variable linker between the phosphoester and the cationic entity. The first was based on the treatment of a fatty dialkylchlorophosphate (obtained from POCl3 and two equivalents of fatty alcohol) with phosphorous and arsenic ylides, which resulted, after acidification, in the phosphonium and arsonium methylenephosphonates. Tri-methylsilyl stabilized ylides... 

In early work, certain seven-membered and larger cyclic arsines were characterized by preparation and isolation of their arsonium salts by quaternization with alkyl halides. Reaction of more elaborate seven-membered cyclic arsines with activated alkyl bromide is shown in Equation (1). The C2-symmetric chiral tertiary arsine 61 reacts with methyl bromoacetate in a pressure tube under microwave irradiation to give the arsonium salt 62 in 71%. The latter was employed in the enantioselective olefination of 4-substituted cyclohexanes via the corresponding stabilized ylide <2002TA2187>. 

Equilibrium concentrations of semi-stabilized benzylidene ylides derived from optically pure arsonium salts and alkali metal ethoxides react with aromatic aldehydes to produce high yields of enantiomerically enriched tra s-2,3-diaryloxiranes and optically pure tertiary arsines with retention of configuration at arsenic (equation S)" -" . 

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